The invention relates to a tensioning device for a belt drive that has an endlessly revolving belt, an electric machine with a machine housing and a drive wheel and at least one additional drive wheel that is in drive connection with the drive wheel via the belt, wherein the tensioning device comprises the following:                a tensioner housing that is supported by means of a slide bearing on the machine housing so that it can pivot about the axis of the drive wheel,        two tensioning rollers that apply a pre-tensioning force to the belt before and after the drive wheel in the direction of revolution of the belt,        a spring means generating the pre-tensioning force,        and a tensioning arm supported against the force of the spring means so that it can move in the tensioner housing,wherein one of the tensioning rollers is supported on the tensioning arm and the other tensioning roller is supported fixed in place on the tensioner housing.        
The invention also relates to an electric machine with such a tensioning device.
For belt drives with drive wheels receiving and producing alternating torque and a corresponding change from the tight belt span to the slack belt span, the tensioning of the slack belt span requires a tensioning device with two tensioning rollers that pre-tension the traction element before and after the alternating driving and driven drive wheel. This typically by not necessary involves the belt drive of an internal combustion engine with an electric machine in the form of a starter generator that is not only driven by the internal combustion engine for the purpose of generating electricity, but also drives the internal combustion engine, in order to start this internal combustion engine.
Known tensioning devices, like those disclosed, for example, in DE 199 26 615 A1, DE 10 2008 025 552 A1, and DE 10 2006 019 877 A1, each have a tensioner housing mounted on the starter generator and two tensioning arms that are supported so that they can move within this housing, with force being applied onto the tensioning rollers of these tensioning arms by a spring element that is arranged in-between, in order to tension the belts.
A tensioning device of the type named above, i.e., with only one tensioning arm that is supported so that it can move in the tensioner housing on one side and a tensioning roller that is fixed on the housing on the other side, and an electric machine with such a tensioning device are known from the unpublished DE 10 2010 048 206 A1.